This invention relates to an optical fiber submarine cable structure in the field of optical communication employing low-loss optical fibers as transmission media.
As an optical fiber is made of a glass or silica material and is a very fine thread, it is given a coating for increased tensile strength necessary for use in a cable form and for protection against external forces. The surface of this coating has fine unevenness; therefore, when the optical fiber is exposed to an external force, such as static hydraulic pressure, a force applied to the optical fiber in its diametrical direction becomes nonuniform with respect to its lengthwise direction, causing slight bending of the optical fiber. This exerts a bad influence upon the transmission characteristic of the optical fiber to increase a transmission loss. In a case where such an optical fiber is laid on the seabed of several thousand meters as an optical fiber submarine cable, the abovesaid static hydraulic pressure attains up to several hundred atmospheres; and in addition to a problem of the coating of the optical fiber, it is a serious problem whether or not a stable transmission characteristic can be maintained for such a long time such as more than 20 years. In view of the above, various pressure resisting layers have heretofore been proposed for protecting the optical fiber from high hydraulic pressures in submarine cable structures.
The present inventors have previously proposed a small-diametered, thin pipe based on the small diameter of the optical fiber or a pressure resisting layer of such a construction in which grooves are formed in a metal columnar structure and closely covered with a lid, or in which divided thick cylindrical members are assembled together so that the pressure resisting layer is used to perform not only its own function but also the function of a power-feeding member or a tension member. In any of these proposed structures, cylindrical members are divided into segments and, for maintaining airtightness of the pressure resisting cylindrical structure over such a long distance as a few dozen kilometers, a high degree of material working technique is required and the manufacture of the pressure resisting layer is difficult.